A conventional lens tube incorporates one lens group or a plurality of lens groups as a photographing optical system. The lens tube is generally assembled from a plurality of lenses, which constitute such a lens group, and a lens frame which incorporates and holds the lenses. A method of assembling the lens tube shown in FIG. 13 is known as a method for incorporating a plurality of lenses, which belong to the lens group of the lens tube, into the lens frame at a predetermined distance. In this method of assembling the lens tube, a lens tube 105 is formed by incorporating lenses 101, 102 into a lens frame 103 from one direction and interposing a space ring 104 which is processed in advance to have a predetermined thickness so that a predetermined distance may be secured between the lenses 101 and 102.
However, in the method of assembling the lens tube shown in FIG. 13, clearance is needed between the inner wall of the lens frame 103 and the space ring 104, and therefore when the space ring 104 is decentered, the point of contact between the space ring 104 and the lens 101 (or the lens 102) is displaced and the lens 101 (or the lens 102) may be inclined as a result.
Accordingly, a lens tube with the configuration as shown in FIG. 14A and FIG. 14B has been proposed for the purpose of preventing such inclination of the lenses (see JP H9-318858 A). It is to be noted that FIG. 14A(a) is a transverse cross-sectional view seen from XIVAa-XIVAa line of FIG. 14A(b), FIG. 14A(b) is a vertical cross-sectional view seen from XIVAb-XIVAb line of FIG. 14A(a), FIG. 14B(a) is a transverse cross-sectional view seen from XIVBb-XIVBb line of FIG. 14B(b), and FIG. 14B(b) is a vertical cross-sectional view seen from XIVBa-XIVBa line of FIG. 14B(a).
As shown in FIG. 14A (FIG. 14B), the lens tube uses at least three steel balls 206 (306) as component members equivalent to conventional space rings for securing a predetermined distance between lenses 201, 202 (301, 302) within a lens frame 203 (303), and these steel balls 206 (306) are placed at equidistant positions obtained when the lens 201, 202 (301, 302) are generally trichotomized in the circumferential direction. Accordingly, assembling is conducted with a distance between the lenses 201, 202 (301, 302) set at a predetermined value based on the diameter size of the steel ball 206 (306).
Since the variation in the outline dimension of commercially available steel balls is several micrometers or some tenths of one micrometer, it becomes possible to set the precision error with respect to the distance between lenses at several micrometers or less than some tenths of one micrometer.
The assembly process in the lens tube assembling method employs the steps for first inserting the lens 201 (301) into the lens frame 203 (303), then placing the steel balls 6 within the lens frame 203 (303), and finally inserting the lens 202 (302) into the lens frame 203 (303).
The lens frame 203 (303) of the lens tube has a positioning structure 207 (207) having a recess section, which regulates the steel balls 206 (306) in the circumferential direction and in the diameter direction so as to position the steel balls 106 at equal intervals.
However, in the case where the positioning structure 207 (307) is provided in the lens frame 203 (303) as seen in FIG. 14A and FIG. 14B, manufacturing costs of the tube increase with complication of the lens frame configuration.
In the configuration as seen in FIG. 14B, it is necessary to provide the positioning structure 307 inside the steel ball 306, which reduces the effective area of the lens.
In order to secure a wider effective area of the lens, the inside diameter of the positioning structure needs to be expanded, and this requires the inside diameter of the lens frame to be expanded, which in turn causes increase in tube size.
For example, in the case where the lens tube is used as a component member of a camera module for cellular phones which are required to be downsized, it is preferable to decrease the size of the tube as a component member of the module since upsizing of the tube leads to upsizing of the entire module.
When the positioning structure is not formed in the lens tube, it becomes possible to secure a larger effective area and to constitute a lens system with a combination having higher flexibility. However, when steel balls are placed at predetermined positions in the circumferential direction, the motion of the steel balls is not regulated, and therefore the steel balls may move from the predetermined positions during the assembling process, which may generate clearance between the steel balls and the lens frame inner wall and eventually cause inclination of the lenses as in the case of using the space rings.
In the case where the surface on which the steel balls 406 are placed is flat as shown in FIG. 15 in particular, the motion of the steel balls 406 is not regulated in the circumferential direction nor in the diameter direction, and therefore it is extremely difficult to place the steel balls 406 at generally trichotomized equidistant positions so as to be in contact with the inner wall of the lens frame 403 and then to conduct assembling while keeping the steel balls 406 unmoved.
Moreover, in the case where the surface on which the steel balls 406 are placed is concaved, the motion of the steel balls 406 is not regulated in the circumferential direction nor in the diameter direction, and therefore it also becomes difficult to proceed the assembling process afterward as in the case of the flat surface.